The present invention relates to a magnetron control arrangement.
Microwave heating is a technique which can be applied very advantageously in a number of processes which require the supply of thermal energy. One important advantage is that the heating power can be controlled in the absence of any inertia.
One drawback, however, is that microwave equipment is often more expensive than other conventional alternatives. Heating equipment includes, inter alia, a power unit with associated control system for driving the magnetron. Such a power unit and associated control system is responsible for the predominant part of the cost of such equipment. Since the power output of the magnetron is limited, it is often necessary to provide a significant number of magnetrons with associated power units and control systems to satisfy a given heating requirement.
Two types of magnetron are found, namely magnetrons whose magnetic fields are generated by a permanent magnet and magnetrons whose magnetic fields are generated by an electromagnet.
The strengths of the permanent magnets varies in manufacture and during operation. The magnetron construction includes a magnetic yoke, the permeability of which varies with temperature. Together with geometrical changes which occur with changes in temperature in the magnetron, changes also occur in the characteristic curve, seen as a graph in which anode voltage is plotted against anode current. The power output is proportional to the anode current, with a good degree of accuracy.
These facts are the reason why a multiple of magnetrons cannot be driven directly by a common voltage unit. The graph, or curve, exhibits a knee, the so-called knee voltage, above which the power output of the magnetron is greatly increased.
When two or more magnetrons are connected in parallel to a power unit and the magnetrons have slightly different characteristic curves, which is usually the case, one of the magnetrons will have a higher power output than the other. The magnetron which has the higher power output will become hotter than the other, causing the characteristic curve to fall so that the power unit produces a lower output voltage. In turn this causes the magnetron producing the lower power output to produce still less power, and so on until only one magnetron produces all power, because the knee voltage of the other magnetron is not reached.
One problem is therefore that each magnetron must be controlled individually, while at the same time endeavoring to reduce the number of power units with associated control systems.
A solution to this problem is disclosed in Swedish Patent Specification No. . . . (Swedish Patent Appln No 8602990-7), which solution is characterized in that two or more magnetrons are connected in parallel to a power unit for generating high voltage for operating the magnetrons; in that a separate regulating circuit for each magnetron is connected to respective magnetrons and includes measuring means for measuring the anode current through respective magnetrons on the high-voltage side of the magnetron; in that the measuring means is separated galvanically from a control circuit which is intended to control the anode current of the magnetron concerned in response to a signal from the measuring means.
Thus, according to this patent, the anode current is measured on the high voltage side of respective magnetrons. This means, among other things, that the measuring means must be separated galvanically from the control circuit.
One sound reason for measuring the anode current on the high-voltage side of the magnetron is because the anode of the magnetron is therewith directly earthed. Should the anode current simply be measured on the low-voltage side, the magnetron could be raised up to a high potential, which would be unacceptable from the aspect of safety.
However, it would be advantageous to be able to measure the anode current on the low-voltage side, since this would avoid the problem of separating the measuring circuits from the high operating voltage.